This invention relates to an improved photosensitive element for use in electrophotography and to a method of preparing such a photosensitive element.
Recently, a method of electrophotography has been developed comprising the steps of preparing a photosensitive element including a photoconductive layer having a plurality of charge trapping levels and manifesting persistent internal polarization and a highly insulative layer integrally bonded to the photoconductive layer, applying a first electric field across the photosensitive element to deposit a charge of one polarity on the surface of the highly insulative layer and applying a second electric field across the photosensitive element to deposit a charge of the opposite polarity on the surface of the highly insulative layer concurrently with the projection of a light image upon the photosensitive element, thus forming an electrostatic latent image on the surface of the highly insulative layer corresponding to the light image. This method is superior to other methods in that it is possible to use photoconductive materials of such low dark resistance that they should not be used in prior methods of electrophotography, thus providing extremely high photosensitivity. This method and photosensitive elements and materials useful in the method are fully described in U.S. Pat. Nos. 3,457,070 and 3,536,483 assigned to the assignee of the present invention.
According to this method, however, trapping of charge carriers at the interface between the highly insulative layer and the photoconductive layer or the surface layer thereof close to the highly insulative layer is utilized to form the latent image and it is necessary to use materials having a large number of charge trap levels for the photoconductive layer. Photoconductive photosensitive materials having a large number of trap levels or impurity levels generally have low photosensitivity. For this reason, it has been difficult to provide an element having both properties of efficient charge trapping and improved photosensitivity.